Inductive Charger with Magnetic Shielding

ABSTRACT

To recharge an implanted medical device, an external device, typically in the form of an inductive charger, is placed over the implant to provide for transcutaneous energy transfer. The external charging device can be powered by a rechargeable battery. Since the battery is in close proximity to the charge coil, the large magnetic field produced by the charge coil induces eddy currents that flow on the battery&#39;s metallic case, often resulting in undesirable heating of the battery and reduced efficiency of the charger. This disclosure provides a means of shielding the battery from the magnetic field to reduce eddy current heating, thereby increasing efficiency. In one embodiment, the magnetic shield consists of one or more thin ferrite plates. The use of a ferrite shield allows the battery to be placed directly over the charge coil as opposed to outside the extent of the charge coil.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/040,945, filed Mar. 4, 2011, which was a non-provisional ofU.S. Provisional Patent Application Ser. No. 61/318,143, filed Mar. 26,2010. Priority is claimed to these applications, and both areincorporated herein by reference in its entireties.

FIELD OF THE INVENTION

The present invention relates to an improved wireless external chargerfor more efficiently charging an implantable medical device, wherein theexternal charger comprises a magnetic shield. The invention alsoprovides an integrated and compact, low-profile charger design thatcontains a battery, coil, and magnetic shield, all co-axially aligned ina single, self-contained housing.

BACKGROUND

Implantable stimulation devices generate and deliver electrical stimulito body nerves and tissues for the therapy of various biologicaldisorders, such as pacemakers to treat cardiac arrhythmia,defibrillators to treat cardiac fibrillation, cochlear stimulators totreat deafness, retinal stimulators to treat blindness, musclestimulators to produce coordinated limb movement, spinal cordstimulators to treat chronic pain, cortical and deep brain stimulatorsto treat motor and psychological disorders, and other neural stimulatorsto treat urinary incontinence, sleep apnea, shoulder sublaxation, etc.The present invention may find applicability in all such applications,although the description that follows will generally focus on the use ofthe invention within a Spinal Cord Stimulation (SCS) system, such asthat disclosed in U.S. Patent Publication 2007/0038250.

Spinal cord stimulation is a well-accepted clinical method for reducingpain in certain populations of patients. An SCS system typicallyincludes an Implantable Pulse Generator (IPG), electrodes, at least oneelectrode lead, and, optionally, at least one electrode lead extension.As shown in FIG. 1, the electrodes 106, which reside on a distal end ofthe electrode lead 102, are typically implanted along the dura 102 ofthe spinal cord 104, and the IPG 100 generates electrical pulses thatare delivered through the electrodes 106 to the nerve fibers within thespinal column 104. Electrodes 106 are arranged in a desired pattern andspacing to create an electrode array 110. Individual wires 112 withinone or more electrode leads 102 connect with each electrode 106 in thearray 110. The electrode lead(s) 102 exit the spinal column 104 and mayattach to one or more electrode lead extensions 119 a and 119 b. Theelectrode lead extensions 119 a and 119 b, in turn, are typicallytunneled around the torso of the patient to a subcutaneous pocket wherethe IPG 100 is implanted. Alternatively, the electrode lead 102 maydirectly connect with the IPG 100.

As should be obvious, an IPG needs electrical power to function. Suchpower can be provided in several different ways, such as through the useof a rechargeable or non-rechargeable battery or through electromagnetic(EM) induction provided from an external charger, or from combinationsof these and other approaches, which are discussed in further detail inU.S. Pat. No. 6,553,263. Perhaps the favorite of these approaches is touse a rechargeable battery in the IPG, such as a Lithium-ion battery ora Lithium-ion polymer battery. Such a rechargeable battery can generallysupply sufficient power to run an IPG for a sufficient period (e.g., aday or more) between recharging. Recharging can occur through the use ofEM induction, in which EM fields are sent by an external charger to theIPG. Thus, when the battery in the IPG needs recharging, the patient inwhich the IPG is implanted can activate the external charger totranscutaneously (i.e., through the patient's flesh 114) charge thebattery (e.g., at night when the patient is sleeping or during otherconvenient periods). In FIG. 1A, the external charger is representedgenerically by coil 108, which coil can be used to produce an EM field110 capable of transcutaneous transmission through the patient's flesh114.

Several basic varieties of external charger designs possessing acharging coil (such as coil 108) have been disclosed in the prior art.See, e.g., U.S. Patent Publication 2009/0118796; U.S. Patent Publication2010/0204756; and U.S. Patent Publication 2008/027500. The operation ofthese prior art external chargers function essentially as shown in FIG.2. As shown, the system comprises, in relevant part, the externalcharger 158 and IPG 100. A primary coil 108 in the charger 158 producesan EM field 110 capable of transcutaneous transmission through apatient's flesh 114. The EM field 110 is met at the IPG 100 by anothercoil 200, and accordingly, an AC voltage is induced in that secondarycoil 200. This AC voltage in turn is rectified to a DC voltage at arectifier 202, which may comprise a standard bridge circuit. (There mayadditionally be data telemetry associated with the EM field 110, butthis detail is ignored as impertinent to the present disclosure). Therectified DC voltage is, in turn, sent to a charge controller andprotection circuit 204, which operates generally to regulate the DCvoltage and to produce either a constant voltage or constant current,Ibat, output as necessary for recharging the IPG 100's internalrechargeable battery 206. Further details concerning external chargerscan be found in the '955 application.

As shown in FIG. 3, electrical current flowing into the page at thelower end of coil 108 and out of the page at the upper end of coil 108induces a magnetic field 110 having a prominent portion in a directionperpendicular to the plane in which the primary coil 108 lies. Primarycoil 108 is typically formed of many turns of copper Litz wire, of whichonly a handful of individual turns are shown in FIG. 3 for clarity.Thus, when a face of the case of the external charger 158 is oriented inclose proximity to an implanted device, such that the primary coil 108is parallel to a corresponding secondary coil 200 within the IPG 100,the magnetic field generated by the primary coil 108 induces anelectrical current within corresponding coil 200 to charge the battery214 within, or otherwise provide power, to the IPG 100.

As shown in FIG. 3, the magnetic field generated by an unshieldedprimary coil generates a magnetic field which is in part directed towardthe secondary coil where it performs useful work, and which is in partdirected away from the secondary coil where the magnetic field energy issubstantially wasted. If a higher percentage of the magnetic field fromthe primary coil could be directed to the implanted secondary coil, theenergy required to drive the external charger could be reduced, whichcould allow the external charger to be made smaller. One such method ofdirecting a higher percentage of the magnetic field from the primarycoil towards the body is to use a magnetic field shield behind theprimary coil's windings, such as is illustrated in U.S. Pat. No.6,389,318. Such a design can enhance the energy transferring efficiencyof the external charger/implantable device system by reflecting magneticfield lines back inwards. The magnetic field shield can be constructedof any material with a high permeability, such as, but not limited to,ferrite powder or ferrite plates.

Heretofore, attempts at producing a wireless and integrated (i.e.,containing a power source, charging coil, and associated charging and/ortelemetry circuitry in a single, self-contained package), compact, andlow-profile external charger were complicated by the generation ofexcessive heating and eddy currents in the casing of the externalcharger's power source, usually a rechargeable battery. In the state ofthe art charging device, the external charger's battery is placed nearthe charging coil inside the charging device. Due to this closeproximity, the magnetic field produced by the charge coil induces eddycurrent heating in the battery case. This has the undesirable effect ofboth additional device heating as well as reduced charging efficiency.As much as 20% of the power transmitted by the charge coil is lost dueto this coupling between the charge coil and the battery.

Given these shortcomings, the art of implantable devices would benefitfrom an improved wireless external charger design that is integrated,compact, and low-profile, that also comprises a magnetic shield. Such acharger would provide for: increased charging efficiency; fastercharging rates; increased patient safety and comfort; lower powerrequirements; and a smaller form factor. This disclosure presents asolution to this problem, disclosing an external charger comprising: ahousing; a coil within the housing; a rechargeable battery within thehousing; and a magnetic shield within the housing comprising a plate orplates made of a high permeability material, wherein the magnetic shieldis located between the battery and the coil, wherein the coil, battery,and magnetic shield are co-axially aligned, and wherein the coil is usedto provide power to an implantable medical device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an implantable pulse generator (IPG), an external chargingcoil, and the manner in which an electrode array is coupled to the IPG,in accordance with the prior art.

FIG. 2 illustrates a prior art system comprising an external charger forcharging an implantable pulse generator, including the charge controllerand battery protection aspects of the IPG.

FIG. 3 shows a side view of representative magnetic field linesgenerated by a prior art external charger for an implantable medicaldevice.

FIG. 4 shows one embodiment of an improved external charger for chargingan implantable pulse generator

FIGS. 5A and 5B show side and bottom views of one possible embodiment ofan improved external charger for an implantable medical device whereinthe magnetic shield comprises one or more tiled ferrite plates.

FIG. 6 shows a side view of representative magnetic field linesgenerated by the external charger of FIGS. 5A and 5B.

FIGS. 7A and 7B show side and bottom views of one possible embodiment ofan improved external charger for an implantable medical device whereinthe magnetic shield comprises a battery-enclosing box constructed of oneor more ferrite plates.

FIG. 8 shows a side view of representative magnetic field linesgenerated by the external charger of FIGS. 7A and 7B.

DETAILED DESCRIPTION

The description that follows relates to use of the invention within aspinal cord stimulation (SCS) system. However, it is to be understoodthat the invention is not so limited. Rather, the invention may be usedwith any type of implantable medical device that could benefit fromimproved charging between an external charger and the implantabledevice. For example, the present invention may be used as part of asystem employing an external charger configured to charge a pacemaker,an implantable pump, a defibrillator, a cochlear stimulator, a retinalstimulator, a stimulator configured to produce coordinated limbmovement, a cortical or deep brain stimulator, or in any otherstimulator configured to treat urinary incontinence, sleep apnea,shoulder sublaxation, etc. Moreover, the technique can be used innon-medical and/or non-implantable devices or systems as well, i.e., inany device or system in which improved coupling between a primary andsecond device is necessary or desirable.

To recharge an implanted medical device, an external device, typicallyin the form of an inductive charger, is placed over the implant toprovide for transcutaneous energy transfer. The external charging devicecan be powered by a rechargeable battery. Since the battery is in closeproximity to the charge coil, the large magnetic field produced by thecharge coil induces eddy currents that flow on the battery's metalliccase, often resulting in undesirable heating of the battery and reducedefficiency of the charger. This disclosure provides a means of shieldingthe battery from the magnetic field to reduce eddy current heating,thereby increasing efficiency. In one embodiment, the magnetic shieldconsists of one or more thin ferrite plates. The use of a ferrite shieldallows the battery to be placed directly over the charge coil as opposedto outside the charge coil. In another embodiment, the magnetic shieldconsists of a battery-enclosing box consisting of one or more thinferrite plates. The use of a ferrite box allows the battery to be placedcompletely within the extent of the charge coil.

FIG. 4 shows one embodiment of an improved external charger 400 forcharging an implantable device that is integrated, compact, low-profile,wireless, and contains a battery, coil, and magnetic shield, allco-axially aligned within the single, self-contained housing. Theexternal charger 400 is shown sitting in a base unit 404 that may beused for charging the external charger 400. In this embodiment, fourarrow-shaped LED lights 402 are arranged on the surface of the externalcharger 400, with one arrow-shaped LED light pointing towards each edgeof external charger 400. The LED lights 402 can, in someimplementations, be used to help the patient better align the externalcharger 400 with the implantable device 100, as is explained further inU.S. Patent Publication 2011/0004278.

FIGS. 5A and 5B show side and bottom views, respectively, of theinternal components of one possible embodiment of an improved externalcharger 500 for an implantable medical device that is similar in formfactor to the external charger 400 shown in FIG. 4. Charger 500 is anintegrated and compact, low-profile, wireless external charger designthat contains a battery 514, a coil 108, and a magnetic shield 504, allco-axially aligned, in a single stack. Magnetic shield in thisembodiment comprises one or more tiled ferrite plates 504. As shown inFIG. 5A, the external charger 500 also consists of a case or housing510, typically formed of a hard plastic, which may be divided into tophalf 510 a and bottom half 510 b along a central axis 512. Clamps 502may be utilized to hold a printed circuit board 506 in placemechanically. Clamps 502 are shown formed as a part of the top casehalf, although this is not strictly necessary, as other means can beused to stabilize the components within the case 510. Associatedelectronic circuitry 508 may be printed onto PCB 506 in any desiredlocation, but preferably behind magnetic shield 504 so as to minimizethe generation of any eddy currents in the associated electroniccircuitry 508. Battery 514 can be placed on the opposite side of PCB 506from the coil 108. A thin prismatic battery rather than a cylindricalbattery can be used in the charger, allowing for a low-profile chargerpackage. For example, Lithium-ion battery Model No. CGA633450B fromPANASONIC® provides a 3.7V/1200 mAh power source that has dimensions of34.0 mm wide, 50.0 mm long, and just 6.3 mm thick.

Since the battery 514 in the charger 500 depicted in FIGS. 5A-5B is inclose proximity to the charge coil 108, the large magnetic fieldproduced by the charge coil would, in the absence of a magnetic shield,tend to induce eddy currents to flow on the battery's 514 metallic case,which is typically constructed of aluminum or steel. These eddy currentsact to oppose the magnetic field produced by coil 108 and createunnecessary heating of the battery 514 as well as reduced efficiency ofthe charger 500. Therefore, one embodiment of an improved externalcharger 500 design provides a means of shielding the battery from themagnetic field to reduce eddy current heating, thereby increasingefficiency of the charger 500. In the depicted embodiment, the magneticshield comprises one or more ferrite plates 504, but preferably four tosix plates. The ferrite plates can have any shape, although preferablyare square or rectangular to allow for placement in a tiled pattern. Asshown in FIG. 5B, the magnetic shield may consist of four square plates,504 a-504 d. The ferrite plates 504 can be, for example, Model No.HP1040-100 from LAIRD TECHNOLOGIES®, which measure 26.42 mm to a sideand are 1.27 mm thick. Each plate also has an adhesive backing thatallows for easy and simple application to a surface, such as PCB 506, ifso desired. The gaps 516 between the plates 504 are preferablyrelatively small, ideally less than 1 mm, so as to prevent significantflux leakage through the gaps. The primary advantage of having severalsmall plates rather than one large plate is a smaller plate isstructurally stronger than one large plate, as ferrite is somewhatbrittle. Also, the cost to manufacture multiple small plates may belower than the costs to manufacture a single, large plate, especiallywhen the ferrite used is very thin. Alternatively, the ferrite plates504 could comprise “78 Material,” such as that produced by FAIR-RITE®Products Corp., which is a MnZn (Manganese-Zinc) ferrite specificallydesigned for power applications at frequencies up to 200 kHz. In thecurrent application, charging of the implant is preferably done in arange between 80 kHz and 120 kHz, and, thus, 78 Material is an excellentchoice due to its high permeability (approximately 2,000) in thisfrequency range.

The charge coil 108 can then be adhered to the ferrite plates 504, withthe battery 514, as mentioned before, placed on the opposite side of thePCB 506. In other embodiments (not shown), the ferrite plates can beplaced on the side of the PCB 506 where the battery is located, oppositethe side of coil 108. The use of a ferrite shield 504 also allows thebattery 514 to be placed directly over the charge coil 108 as opposed tooutside the extent of the charge coil 108.

Without the magnetic shield comprising the tiled ferrite plates 504a-504 d, the external charger 500 would experience significant decreasesin charging efficiency. Because battery 514 has a metal casing, eddycurrents would be generated in the battery casing by the inducedmagnetic field 110 of coil 108, and such eddy currents would create anopposing magnetic field. These eddy currents will result in energy beingtransferred to the metal battery case in the form of heating losses.Thus, the external charger would be losing efficiency because of thepower being dissipated in the battery 514's case. This would cause theimplanted medical device 100 to not charge as quickly, and/or cause thebattery 514 in the external charger 500 to deplete faster While themagnetic field induced by the eddy currents may only be on the order ofabout 5% of magnetic field 110 induced by coil 108, these negativeeffects would still be significant. Additionally, the heat resultingfrom the eddy currents is unwanted.

However, when the ferrite plates 504 are used, these negative effectsare lessened. As shown in FIG. 6, the increased permeability of theferrite plates 504 causes the magnetic field lines inside the ferrite toflow in a plane parallel with the coil 108 instead of perpendicular tothe coil 108, thus diverting the magnetic field from reaching thebattery 514. Additionally, the ferrite plates 504 increases the overallefficiency of the charging system due to fewer magnetic field linesbeing directed away from the patient's body, where the magnetic fieldenergy is substantially wasted (compared FIG. 3). Due to the placementof magnetic shield 504, a higher percentage of the magnetic field 110from the primary coil 108 is directed towards the implanted secondarycoil (not shown) across skin boundary 114. The relative permeability ofthe ferrite shield, which is typically in the range of 500 to 5000, canincrease the quality factor of the coil 108 by as much as 50% due atleast in part to the “reflection” of magnetic flux back towards thepatient's implanted device.

FIGS. 7A and 7B show side and bottom views of an alternative embodimentof an improved external charger 700 for an implantable medical devicethat is similar in form factor to the external charger 400 shown in FIG.4. In this embodiment, the magnetic shield comprises a battery-enclosingferrite box 702 constructed of a material with good magnetic shieldingproperties, i.e., a material with high permeability such as thosediscussed earlier. In this alternative embodiment, the ferrite box 702covers the battery 514 from all sides. However, because the box may bemade with plates as in the earlier embodiment, the ferrite box 702 maycontain small gaps which would not significantly alter theirfunctionality.

External charger 700 is similar in design to external charger 500 ofFIGS. 5A-5B. However, in contrast to the design of external charger 500,and as just mentioned, the battery 514 of external charger 700 is placedinside the ferrite box 702. The ferrite box 702 can be composed of twoclamshell plates that enclose the battery, e.g., 702 a and 702 b, ormultiple plates arranged in a tiled pattern that enclose the battery onall sides. One advantage of this embodiment is that the housing 710 canbe made thinner since the charge coil 108 does not lie on the ferritebox 702 (compare FIGS. 5A and 5B), but rather surrounds it. Onepotential disadvantage, however, is that the ferrite box 702 canincrease the overall weight of the charger, but this can be a suitabletrade off. Also, the area of PCB 704 is somewhat reduced as a cutoutarea 706 needs to be made in the center of the PCB 704 to accommodatethe battery 514 and ferrite box structure 702. Coil 108 can comprise acoil that is wrapped in a racetrack, planar configuration around theouter edge of the PCB 704, with the associated electronic circuitry 508printed onto the side of the PCB 704 opposite the coil 108. Again, theuse of a thin prismatic battery rather than a cylindrical battery in thecharger allows for a low-profile external charger housing.

FIG. 8 shows a side view of representative magnetic field lines 110generated by the external charger 700 of FIGS. 7A and 7B. Similar toFIG. 6, it can be seen that the ferrite box 702 increases the overallefficiency of the charging system due to fewer magnetic field linesbeing directed backwards and away from the patient's body, where themagnetic field energy is wasted. A higher percentage of the magneticfield 110 from the primary coil 108 is directed towards the implantedsecondary coil across skin boundary 114, though the effect is not asgreat as that seen in FIG. 6 due to the particular arrangement requiredto accommodate the ferrite box 702 within the extent of the coil.However, the ferrite box 702 does provide for superior shielding of thebattery 514 from eddy currents created by field lines 110 in the battery514's casing.

Although particular embodiments of the present invention have been shownand described, it should be understood that the above discussion is notintended to limit the present invention to these embodiments. It will beobvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present invention. Thus, the present invention is intended to coveralternatives, modifications, and equivalents that may fall within thespirit and scope of the present invention as defined by the claims.

What is claimed is:
 1. An external device for use with an implantablemedical device, comprising: a housing; a coil within the housing; abattery within the housing; and a magnetic shield within the housingcomprising a high permeability material, wherein the coil, the magneticshield, and the battery are aligned in a stack, wherein the magneticshield is located between the battery and the coil in the stack, andwherein the coil generates a magnetic field to provide power to theimplantable medical device.
 2. The device of claim 1, wherein themagnetic shield comprises a ferrite plate.
 3. The device of claim 1,wherein the battery comprises a rechargeable Lithium-ion battery.
 4. Thedevice of claim 1, wherein the magnetic shield covers the entire extentof the coil.
 5. The device of claim 1, wherein the coil, battery, andmagnetic shield are co-axially aligned in the stack.
 6. The device ofclaim 5, further comprising a printed circuit board (PCB) within thehousing of the external device.
 7. The device of claim 6, wherein themagnetic shield is adhesively attached to the PCB.
 8. The device ofclaim 6, wherein the magnetic shield and coil are located on a firstside of the PCB, and the battery is located on a second side of the PCB.9. The device of claim 6, wherein the magnetic shield and battery arelocated on a first side of the PCB, and the coil is located on a secondside of the PCB.
 10. The device of claim 1, wherein the magnetic shieldcomprises a plurality of plates.
 11. The device of claim 10, whereineach of the plurality of plates is a ferrite plate, and wherein theplurality of plates are configured in a tiled arrangement thatsubstantially covers the entire extent of the coil.
 12. An externaldevice for use with an implantable medical device, comprising: ahousing; a coil within the housing; a battery within the housing; and amagnetic shield within the housing made of a high permeability material,wherein the magnetic shield covers the battery, wherein the magneticshield and battery are within the extent of the coil, and wherein thecoil generates a magnetic field to provide power to the implantablemedical device.
 13. The device of claim 12, wherein the magnetic shieldcomprises a box.
 14. The device of claim 13, wherein the box comprisestwo clamshell ferrite plates.
 15. The device of claim 13, wherein thebox comprises a plurality of ferrite plates configured in a tiledarrangement that substantially covers the entire battery.
 16. The deviceof claim 12, further comprising a printed circuit board (PCB) within thehousing of the external device.
 17. The device of claim 16, wherein thePCB has a cutout area to accommodate the battery and magnetic shield.18. The device of claim 16, further comprising electronic circuitry,wherein the coil is located around an outer edge of a first side of thePCB, and the electronic circuitry is located on a second side of thePCB.
 19. The device of claim 12, wherein the magnetic shield containsgaps.
 20. The device of claim 12, wherein the coil, battery, andmagnetic shield are co-axially aligned.